Electrospray tandem mass spectrometric studies of all twenty N-phosphoryl amino acids

Cleavage of phosphorus-carbon (P-C) bonds of α-amino phosphonates with intramolecular hydrogen migration in the gas phase using electrospray ionization tandem mass spectrometry

RATIONALE

α-Amino phosphonates with intrinsic biological activities have been used in a wide variety of applications. Because of the widespread existence of natural organophosphorus compounds containing P-C bonds such as the α-amino phosphonates, it is important to investigate the gas-phase chemistry of P-C bonds in order to determine their basic properties, which might provide some insights into their biosynthesis and catalytic cleavage.

METHODS

Twenty α-amino phosphonates were successfully synthesized and their fragmentation behavior was systematically investigated using in-solution deuterium labeling in combination with high-resolution Fourier transform ion cyclotron resonance (FTICR) electrospray ionization tandem mass spectrometry.

RESULTS

The fragmentation pathways of twenty α-amino phosphonates with different chemical structures were systematically studied. In general, P-C bonds could be easily cleaved via a novel intramolecular hydrogen atom migration from the amino group to the phosphoryl group through a five-membered-ring intermediate in the gas phase. A possible mechanism of the rearrangement of α-amino phosphonates is proposed.

CONCLUSIONS

An interesting intramolecular hydrogen atom migration between the amino and phosphoryl groups was observed with cleavage of the P-C bond in the molecule through a five-membered-ring intermediate. This characteristic fragmentation pathway not only provides some insights into the basic chemistry of compounds with P-C bonds, but could also have some applications in the structural determination of the α-amino phosphonate analogues.

A novel hydrogen migration of dialkylphosphonic acid esters using electrospray ionization tandem mass spectrometry

In this paper, attention is focused on analysis of the fragmentation of α-hydroxy-β-amino phosphonate esters designed as inhibitors of protein kinase A. An interesting proton migration mechanism in the cleavage of the P-C bond is investigated by electrospray ionization tandem mass spectrometry. A possible rearrangement mechanism is proposed and verified by high-resolution mass spectra using isotope deuterium/hydrogen-exchange technology and additionally checked by detailed DFT calculation based on Gaussian software. The result clearly indicates that this mechanism proceeds by a five-membered ring concerted transition state with activation energy 11.3 kcal mol(-1) for the compound 3f. The overall reaction is endothermic with an energy 13.2 kcal mol(-1). The effect of different substituents and different metal ions for rearrangement of these esters is studied by experiment and theory. It is concluded that this rearrangement process is energetically unfavorable and hence only occurs in the mass spectrometer.

Study on [(4-substituted benzoylamino)phenylmethyl]phosphonic acid diisopropyl esters under electrospray ionization tandem mass spectrometric conditions

Phosphonate esters were synthesized and investigated by positive ion electrospray ionization mass spectrometry (ESI-MS) in conjunction with tandem mass spectrometry (MS/MS). It was found that for the sodiated adduct there were two novel rearrangement reactions, in which the phosphoryl oxygen atom migrated to the carbonyl carbon and a cleavage occurred at the amide bond, or a benzylamine fragment was lost and the phosphorus was reduced. However, when the methyl group substituted for the isopropyl group, these migrations were inhibited. A possible mechanism was proposed, and high-resolution mass spectrometry was applied to identify the formula of these novel ions. These results showed that ESI-MS is a useful tool for the structural determination of phosphonate esters.